Quaternary ammonium salts such as tetra-n-butyl ammonium bromide (TBAB) are known to form semiclathrate hydrates. Since they form at much milder conditions compared to gas hydrates, they have evoked much interest in development of new technologies for gas storage and gas separations. In this work, we present a method to compute the phase equilibrium of TBAB semiclathrate. The TBAB molecule is modeled using OPLS-AA and GAFF force fields and the results from simulations are compared with experimental data to determine the ability of the force fields to accurately predict the semiclathrate phase equilibria. It was observed that the OPLS-AA force field outperforms the GAFF force field in predicting the experimental phase equilibrium data.